Optical monitoring of physiological characteristics utilizes the detection of light transmitted through a location of a user being measured. Photoplethysmography (PPG) is an optical measurement technique used to detect blood volume changes in the microvascular bed of living tissue, typically by detecting light transmitted through the ear lobe or fingertip. As arterial pulsations enter the capillary bed, changes in the volume of the blood vessels or characteristics of the blood itself modify the optical properties of the capillary bed. A PPG signal is used to measure saturation of peripheral oxygen (SpO2), which is an estimation of the level of oxygen saturation in a fluid, such as blood. The PPG signal can also be used to measure blood pressure.
A device such as a pulse oximeter provides for measuring enhanced optical pulsatile signals emitted by the changes in the volume of blood flowing through a user. The pulse oximeter typically has a pair of small light emitting diodes (LEDs) facing a photodiode/photodetector, with a translucent part of the user's body, usually a fingertip or an earlobe, positioned there between. The LEDs illuminate the tissue (e.g. skin) of the user and the photodetector measures small variations in light intensity associated with changes in perfusion in a catchment volume. An oximeter in such a configuration is typically called a transmittance-type oximeter. The light from the LEDs passes through the tissue and is detected by the photodiode. One LED is red, with wavelength of approximately 660 nanometers (nm), and the other is infrared, with a wavelength of approximately 905, 910 or 940 nm. Absorption at these wavelengths differs significantly between oxyhemoglobin and its deoxygenated form. Therefore, the ratio of oxyhemoglobin to deoxyhemoglobin can be calculated from the ratio of the absorption of the red and infrared light, i.e. the ratio of red light to infrared light absorption of pulsating components at the measuring site.
For transmittance-type oximeters, a cuff or holder is typically provided to function primarily as a holder for the photodiode and also as a shield against ambient light. It has been recognised that having a cuff or a holder/clip typically enforces a fixed orientation of the part of the user's body for measurement. This may be undesirable to a user.
On the other hand, apart from transmittance-type oximeters, there also exist reflectance-type oximeters. For reflectance-type oximeters, the LEDs and the photodiode reside on the same side of the translucent part of the user's body. Light from the LEDs are reflected from the portion to be measured and detected by the photodiode. For reflectance-type oximeters, ambient light can be a significant factor in accuracy of light detection by the photodiode. Thus, reflectance-type oximeters typically still require a cuff or a holder/clip to provide shielding against ambient light from interfering with reflected light from the LEDs. This may be undesirable to a user.
Furthermore, for certain types of reflectance-type oximeters without clips, such as those in patch form, a shield is still required on the base of the oximeter to provide the ambient light shielding.
Further to the above, the inventors have recognised that a typical oximeter has relatively complicated design considerations centering on how to incorporate a power source such that the oximeter can function as a standalone device. This may delay development and increase production costs for oximeters.
In addition, for oximeters, ambient light can interfere with readings in the form of ambient noise. For example, ambient light such as those from bilirubin lamps, fluorescent light, infrared heating lamps and direct sunlight etc. can affect the accuracy of SpO2 readings. As a brief introduction, SpO2 calculation is based on the AC and DC components of both a red (RED) and an infra-red (IR) PPG signal. The Red PPG signal is obtained when a red LED of e.g. about 660 nm is reflected off the skin of a user. The IR PPG signal is obtained when an IR LED emitting electromagnetic waves of e.g. about 940 nm is reflected off the skin of a user.
Therefore, without removing ambient light interference from the PPG signals, a true reading of Red or IR PPG signals cannot be acquired. This in turn affects the calculation of SpO2. Typically, a third PPG signal, i.e. ambient PPG, is obtained. This ambient PPG signal is a signal obtained by a photodetector when both the IR and Red LEDs are turned off. Typically, an on-the-fly point-by-point subtraction of the ambient signal from the Red and IR PPG signals is carried out. However, it has been recognised by the inventors that such processing is typically power consuming. Digital reconstruction of PPG signals can also be time consuming. It has also been recognised by the inventors that the above processing may further typically require additional and relatively complicated multi-path circuitry, e.g. a different signal path for Red, Infrared and/or ambient PPG signals.
Thus, in view of the above, there exists a need for an optical measurement device and method that seek to address at least one of the above problems. There also exists a need for a noise cancellation method and system that seek to address at least one of the above problems.